This invention relates to high energy lasers and more particularly to techniques and apparatus for separating reciprocal path optical signals.
When focusing very high energy laser beams on distant targets, it is often necessary to measure distortions in the optical signal returning from the target on the same, or nearly the same, optical path as the outgoing high energy beam in order to measure disturbances in the beam path. The resultant beam path information may be used to control adaptive optic surfaces for the purposes of improving the focal pattern of the target which is otherwise degraded by the propagation path disturbances.
In a shared aperture laser beam separator, the outgoing laser beam and return signal from the target share the same aperture, assuring that the beam and signal have travelled over the same path. An ideal shared aperture separator for reciprocal path beams will direct substantially all of an input laser beam toward the target, i.e. without any internal dissipation or energy dump, and will accept and decouple radiant power of the same wavelength that has been reflected from the target back along exactly reciprocal atmospheric paths.
No real aperture element, of course, can decouple light signals that simultaneously share exactly reciprocal paths if there is no distinction of wavelength and polarization. Attempts to arrange for differences in polarity or wavelength of the outgoing beam and returning signal have required the use of materials on which, or through which, the high energy laser beam is transmitted, at a sacrifice of energy or with a deteriorating effect on the material.
Pulsing the high energy laser beam can allow the return signal to return between pulses for detection and analysis without interference. No satisfactory device for such synchronously timed apertures has yet been created.
Exact beam reciprocity may be given up. For example, return waves may be sampled through a distribution of small holes in a high energy beam relay mirror. However, each hole extracts and dumps some energy, so the total area of all the holes must be kept small compared to the area of the reflecting surface. The edges of the holes facing the detector can scatter significant amounts of outgoing laser beam energy into detector channels.
Accordingly, it is an object of this invention to provide for obtaining beam path information while avoiding the above-described problems.
It is another object to so provide, with apparatus and techniques that are simple and use readily available materials.
Other objects and features will be in part apparent and in part pointed out hereinafter.